Using the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we theoretically explore a novel fluorescent sensor molecule (abbreviated as "2") (Sensors Actuat B-Chem. 2018, 263, 585). Because of its symmetry, three stable structures can be located, ie, 2-enol, 2-SPT, and 2-DPT forms in both S 0 and S 1 states. Via comparing the bond lengths and bond angles involved in the hydrogen bonding moieties, we find the dual intramolecular hydrogen bonds should be strengthened in the S 1 state. And based on infrared (IR) vibrational simulations, we further confirm the strengthening dual hydrogen bonds. Upon the photo-excitation process, the charge redistribution via frontier molecular orbitals (MOs) reveals the tendency of excited state intramolecular proton transfer (ESIPT) reaction. In addition, the constructed S 0 -state and S 1 -state potential energy curves demonstrate that the excited state single proton transfer (ESSPT) should be the most supported one from 2-enol to 2-SPT form. In view of the S 1 -state stable 2-SPT and 2-DPT structures as well as the fluorescence peaks of them, we can further confirm the ESSPT mechanism for 2 chemosensor. This work not only clarifies the excited state behaviors of 2 system but also successfully explain the previous experimental phenomenon.